LED-based lamps—popular for their service life, compactness and energy efficiency—have become an acclaimed substitute for incandescent lamps, but not without potential drawbacks. LED light sources, when working, generate profuse heat; the hotter they get, the worse they function and the sooner they break down. Thus, thermal management has been a huge concern of manufacturers of LED luminaries. Heat, when trapped and accumulated inside the relatively small space of an LED light bulb, causes lumen depreciation or even premature failure. To overcome overheating problems suffered by LED light bulbs, a common solution is to provide a heatsink made of an enlarged metallic object with decent thermal conductivity. The heatsink, which is disposed outside the case of the LED light bulb and thus in direct contact with ambient air, brings heat—which is first conductively transferred to the surface of the heatsink—away from the light bulb with the help of radiation and convection. However, the structure is criticized for its potential safety issues and costs. The risk of electric shock gets greater because the metallic object, which is not only thermally but also electrically conductive, is directly exposed to human touch. Moreover, an insulated power supply must be provided—driving up costs due to a stringent demand for safety and consistency of the power supply—because otherwise the presence of a metallic object will prevent the LED light bulb from completing a high voltage test.
Another solution is to cover an aluminum-based heatsink with a plastic layer presumably to prevent electric shock. However, the plastic coating prevents heat in the aluminum alloy from going out because of poor thermal conductivity of plastic materials. Plastic covering, despite its safety bonus, is unacceptable for LED lamps with higher luminous output and more heat that must be effectively steered away.
Yet another solution is to electrically insulate the outer case of a light bulb while enlarging the LED circuit board, which is configured to serve as a conduit both for power and heat. An example is disclosed in an article published on “China LED online” (a blog hosted by Wechat™, which is a mobile-based messaging service widely used in China). The article discloses an LED light bulb, as shown in FIGS. 1 and 2, which comprises an outer case and two circuit boards. The outer case is made of insulating plastic material and includes vent apertures on the top and the bottom of the case. The two circuit boards—larger than usual—are disposed axially inside the outer case and intersect each other perpendicularly. A power driver, electrically insulated by the outer case, is integrally provided on the lower portion of a first circuit board. Heat generated by LED packages is conducted to the circuit boards and then taken away through a convective pathway defined by the outer case and the circuit boards. LED packages—mounted on the circuit boards, which are disposed upright along a longitudinal axis inside the outer case—are thus configured to direct their luminous outputs across a wide angle around the entire bulb. In this design, the role otherwise played by a metallic heatsink in some light bulbs now has to be accommodated by the circuit boards, which do not always do a good job transmitting heat. To cope with the overheating issue, enlarged circuit boards must be provided, which drive cost up. When an oversized circuit board gets very close to or even in contact with the inner surface of the outer case, light beaming from LED packages cannot be well diffused—thus discrete dim spots are observed—to visually resemble incandescent lamps. Moreover, the light bulb does not emit as much luminous output as it should because a significant amount of light is first directed back to the circuit boards, which then reflect the light to the inner surface of the outer case as opposed to going directly, and more productively, to the outer case. Furthermore, when almost the entire space inside the outer case constitutes what is called convective pathway, the convective activity in the light bulb is not as effective as when a more structurally defined pathway is provided. Finally, larger vent apertures must be provided to accommodate the absence of a metallic heatsink and poor thermal conductivity of the circuit boards. In one embodiment, the cross-sectional area of the top aperture is as big as 634 square millimeters and that of the bottom aperture 1500 square millimeters. The apertures—all with a sizable opening—heighten the threat of electric shock because electricity-loaded parts inside the light bulb are inadvertently accessible.